Consider deprecating old models API

**Problem**
When working with controllers, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

No file, test, or entry point is named in the issue. Start by locating the controller error-handling code and the old models API, then determine which API is proposed for deprecation. Done should include an agreed deprecation scope and more descriptive controller errors, with tests or validation identified before implementation.

Written by the indexing model from the issue text.